a private tracker is a torrent tracker where you can leave a torrent seeding for 1month, and when you come back to it 10TBs of data won't have been leeched from you.

When my friend doesn't use is private tracker and uses a public one like thepiratebay it will take him like 10 days to download a torrent, and while he downloads it it will seed to a ratio of like 10.5